Chapter 6 emulsification and

emulsification by surfactants

2006.4.19.

§1. Introduction

1. Emulsion – immiscible liquid phase, multiphase

dispersion, thermodynamics unstable system

(from a few minutes to a few years)– e.g. milk ,

soybean milk, bearnaise, finishing oil etc.

2. Formation

(1) Emulsification – one fluid dispersed in a second

� dispersephase –分散相, inner phase , discontinuous phase

� dispersion medium –分散介质，outer phase ，continuous

phase

� Three types: o/w, w/o, and w/o/w or o/w/o

� two immiscible, pure liquids cannot form emulsion.

(2) Emulsifying agents – the third component – surfactants

3. Properties

(1) Size and aspect of particles

� Macroemulsions: d>400nm, creamy white

� Miniemulsions: 400nm>d>100nm, blue-white

� Microemulsions: 100nm>d>50nm, translucence

� Nanoemulsions: 50nm<d, transparence

(2) Viscosity – Einstein’s eq.

η = η0 (1 + 2.5φ)

η0 – viscosity of outer phase

φ - volume fraction of inner phase

(3) Electric conductivity – outer phase

o/w 》w/o , ionics 》nonioncs

4. Differentiation of emulsion types

(1) Dilution method

(2) Dyeing method

(3) Electric conductivity

(4) Filter paper method

(5) Light reflectance

(6) Fluorescence method

§2. Theoretics and types of emulsions

Conversion in different types of emulsion

o/w ↔ w/o

1. Phase volume ratio

if particles(inner phase) are rigid ball with same

size, then volume fraction of inner phase

φ ≤ 74.02%

� φwater > 74%, then w/o → o/w

� φwater < 26%, then o/w → w/o

monodisperse & rigid ball – polydisperse & elastic

ball(w/o of φoil=99%)

2. Wedge theory – geometry theory Match model

3. Solubility rule – hydrophilicity of surfactants

(1) Hydrophilic surfactants – o/w

(2) Lyophilic surfactants – w/o

4. Effects of wall

(1) Hydrophilic wall (high energy surface) – o/w

(2) Lyophilic wall (low energy surface) – w/o

5. Kinetic theory of emulsion type

Davies(1957)developed a quantitative theory of

emulsion type relating the type of emulsion formed to

kinetics of coalescence(聚集) of the two types of

droplets present: oil droplets & water droplets

Rate of coalescence of the particles:

dv/dt = Ae-E/kT

A – collision factor; E – energy barrier to coalescence

� hydrophilic surfactants: Eoli↑, dv/dt of oil droplets↓,

dv/dt of water droplets↑, to form o/w emulsions

� lyophilic surfactants: Ewater↑, dv/dt of water droplets↓,

dv/dt of oil droplets↑, to form w/o emulsions

6. Powder for emulsification

(1) Condition of emulsification

� V/L Young’s eq. γSV = γSL + γLV cosθ

Spreading S = γSV - γSL - γLV = γLV (cosθ-1) ≥ 0

� O/w Young’s eq. γSO = γSW + γOW cosθW

or γSW = γSO + γOW cosθO

θW+ θO=180º

liquid

vapor

γSLγSV

γSV

θθw

waterθO

oil

γOW

γSWγSO

Spreading SW = γSO - γSW - γOW = γOW

(cosθW-1) ≥ 0

or SO = γSW - γSO - γOW = γOW (cosθO-1) ≥ 0

∴ If γso > γow + γsw then θW ≤ 0, the powder is in water phase

∴If γsw > γow + γso then θo ≤ 0, the powder is in oil phase

Else the powder is adsorbed at O/W interface

0º < θW < 180º or 0º < θO < 180º

(2) According O/w Young’s eq.

γSO = γSW + γOW cosθW or cosθW = (γSO - γSW)/ γOW

� if γSO > γSW, θW < 90º, hydrophilic , mostly in

water phase o/w

� if γSO < γSW, θW > 90º, lyophilic , mostly in oil

phase w/o

� if γSO = γSW, θW = 90º, balance, o/w or w/o , but

unstable

§3. Factors of effect on stability of emulsion

1.Reduces of interfacial tension between two liquids

e.g. 10ml n-octane(正辛烷) is dispersed in water to

0.1 µm emulsion, then its surface area is 300m2,

γL/L= 50.8mJ/m2, then total surface energy=15.24J

if the surfactants are added into the disperse

system, then γL/L ≤ 1 mJ/m2, then total surface

energy ≤ 0.3J.

Dynamic stability ↑

2. Physical nature of the interfacial film

(a) Mechanical stability –with strong lateral intermolecular forces and high film elasticity

(b) Liquid–crystal formation – with a high-viscosity region and a steric barrier to stabilize of emulsion

3. Existence of an electrical or steric barrier to

coalescence on the dispersed droplets

(a) Electrical barrier –ζ-potential

of ionics:

ζ-potential ↑, stability↑

(b) Steric barrier

4. Viscosity of the outer phase - diffusion coefficient

of droplets: D=kT/6πηa

η - viscosity of the outer phase

a – the radius of the droplets

η↑, D↓, stability↑

5. Size distribution of droplets

larger particle have less interfacial surface per unit

volume than smaller droplets, so they are

thermodynamically more stable than smaller ones.

emulsion with uniform size distribution is more

stable.

§4. Emulsifying agent and HLB value

HLB method which was advanced by Griffin in

1949, is the most frequently used method in

the selection of emulsifying agents.

1.HLB value – Hydrophilic-Lipophilic Balance

HLB = Hydrophilic portion/Lipophilic portion

= 0 ~ 40

Paraffin ~ 0, sodium dodecyl sulfate ~ 40

HLB value < 10 lipophilic

HLB value >10 hydrophilic

HLB value for typical nonionic surfactants structures

2. HLB value & application

1 ~ 3 anti-foaming agent

3 ~ 6 w/o emulsifying agents

7 ~ 9 wetting agents

8 ~ 18 o/w emulsifying agents

13 ~15 detergents

15 ~18 solubilizing agents

3. Estimate of HLB value

(1)Nonionics:

HLB = mass fraction of hydrophilic group×20

(a) Fatty acid of many polyhydric alcohols(多元醇)

HLB = 20(1-S/A)

S – saponification number(皂化值) of the ester;

A – acid number(酸值) of the fatty acid in the ester.

e.g. C17H35COOCH2CH(OH)CH2OH

S=161, A=198, then HLB = 3.7

(b) Nonionics of POE & polyol

HLB = (E + P)/5

E – the weight percentage of oxyethylene content

P – the weight percentage of polyol content

e.g. C16H33(OC2H4)10OH

E = (44 × 10+17)/(44 × 10+17+225) = 67.0%;P = 0

HLB = (E+P)/5 = 67.0/5 = 0.67 × 20 = 13.4

(c) Logarithm: HLB=7+11.7ln(MW/MO)

MW or MO- molecular weight of hydro- or lipo-philic

group: MW=441, MO=225, then HLB=14.9

(2) Ionic surfactants –

group numbers could

be calculated based

upon group

contribution accord-ing

to the formula:

HLB = 7 + Σ(group

numbers)

e.g.C12H25SO4Na

HLB=7+38.7-

0.475×12=40

e.g. C16H33(OC2H4)10OH

HLB=7+0.33 ×10+0.5-

0.475 ×16=3.20

(3) Mensuration

(a) behavior in water HLB Range

no dispersibility 1 – 4

poor dispersion 3 – 6

milky dispersion after

vigorous agitation 6 – 8

stable milky dispersion

(upper end almost translucent) 8 – 10

from translucent to clear 10 – 13

clear solution 13 +

(b) Cloud point – POE nonionics 1% aq.

TP ↑, HLB↑, TP>100ºC, HLB>15,

TP ∝ HLB

(4) HLB of a mixyure

HLBmix= ΣfiHLBi =fAHLBA+(1-fA)HLBB

fi – the weight fraction of surfactant i in mixture

4. Emulsifying agents

(1) Synthetical surfactants

(a) Anionics – HLB > 8 , o/w type

(b) Nonionics – HLB < 18, o/w or w/o type

(c) Cationics - a few

(2) Natural surfactants – lecithin(卵磷脂),

cholesterin(胆甾醇) etc

(3) Polymeric emulsifying agents

(4) Powder emulsifying agents

5. Application - selection of surfactants using HLB

value as emulsifying agents

(1) Optimal HLB value for emulsification

(A)Using the data handbook

(a) HLB value for emulsification of some oils

(b) Mixture of oils

HLBmix= ΣFj HLBj= FAHLBA+(1-FA)HLBB

Fj ,HLBj – weight fraction and HLB of j oil.

(B) Mensuration and check

e.g. using the Span-series (HLBSpen-60=4.3) and

Tween-series (HLBTween-80=15), the optimal HLB

value for emulsification

(2) Optimal emulsifying agents

– two principles

(a) The structure of

hydrophobic groups

of emulsifying agents

is close to the oils

(b) The synergistic effect

6. Phase Inversion Temperature (相转变温度PIT)

(1) Disadvantage of HLB method

(a) Surfactants with same HLB value could possess

different emulsifying ability

(b) It makes no allowance for the change in HLB

value with change in the conditions for

emulsification (temperature, nature of oil & water

phase). E.g. POE T↑, HLB↓

(2) PIT – the temperature when the POE is used to

emulsifying agents from o/w emulsion inversion to

w/o

(3) Mensuration of PIT – emulsion : 3-5% emulsifying

agents and o:w volume ratio =1

(4) Emulsion stability

(a) o/w type : PIT – TS =20-60ºC

TS – storage temperature (储藏温度)of emulsion

(b) w/o type : TS – PIT = 10-40ºC

(c) Preparing temperature of emulsificatopn by the PIT

� Preparation at a temperature 2-4ºC below the PIT to obtain a very fine average particles size

� Cooled down to storage temperature( for o/w) to increases it stability

(5) Factors affecting PIT

(a) PIT ∝ HLB selection of emulsion PIT according to their HLB value

(b) Hydrophility of POE ↑, PIT↑(c) Distribution of POE ↑, PIT↑, o/w stability↑

(d) Addition of electrolyte in water phase, PIT↓(e) In oil phase

� addition of nonpolar organics e.g. paraffin PIT ↑

�addition of polar organics

e.g. long chain, PIT ↑short chain, PIT↓

�Mixture of oils

PITmix= Σϕi PITi = ϕA PITA + (1- ϕA) PITB

ϕi – volume fraction of i oil; PITi – PIT of i oil in this emulsifying system

7. Preparation method of emulsion

(1) Addition of emulsifying agents

(a) in water phase

(b) in oil phase

(c) nascent soap(初生皂法)

(d) mixed films (respectively addition of hydrophilic surfactants in water and hydrophobic surfactants in oil phase)

(e) alternately addition of oil and water

(2) Oil-Water mixing

(a) The addition of water to oil - to obtain a very fine average particles size and stable o/w emulsion

(b) The addition of oil to water

(3) Methods of emulsification

(a) Direct emulsification – 70 - 75ºC

(b) Phase inversion emulsification

(4) Equipment of emulsification

(a) Simple agitating - impulse type (推进式), turbo

type(涡轮式)

(b) Homogenizer – high pressure 6.89-34.47 Mpa

(c) Colloid mill (胶体磨) – rotor(转子) and stator(定子), rotate speed = 1000-20000 rpm ，high

shearing force (高切力)

§5. instability of emulsion and demulsification

1. Inversion of emulsion – o/w ↔ w/o

(1) Phase volume ratio

(2) Polyvalent ions

(3) Temperature T>PIT

(4) Addition of electrolytes

o/w → w/oe.g. Inversion of o/w to w/o

by an interface film of sodium

cetyl sulfate (十六烷基硫酸钠)

and cholesterol (胆固醇)upon

addition of polyvalent cations

2. The creaming(分层),

flocculation(絮凝),and

coalescence(聚结) – the

reversible prelude of

emulsion breaking

(a) Creaming – difference

in gravitational

(b) Flocculation - attractive

force between droplets

(c) Coalescence – irreversible

- breaking, phase separation

3. Emulsion breaking(破乳) – to break the interface films

(1) Methods

(a) Physical method

� Heating

� Electrical precipitation

� Ultrasonic

(b) Chemical method

� De-emulsifying agent

� Inorganic acid

� Polyvalent ion to inversion

(2) Mechanism

(a) Displacing – emulsifying agents are displaced

by de-emulsifying agent

(b) Wetting – powder emulsifying agents

(c) Flocculating – cross-linking agents

(d) Collision - deemulsification

(e) Make the interface film to distortion(变形) and

tendering(变脆)

(3) Factor affecting de-emulsification

(a) pH value – e.g. carboxylate surfactant is

unstable at low pH.

(b) Electrolyte

� Concentration – I ↑, ζ↓, stability↓.

� Valenta value – polyvalent ion ↑, stability↓

(c) Temperature - T↑, solubility of surfactants↑,

intensity of film↓

(d) Phase volume ratio – volume fraction > 74.02%

(h) Stirring – Reynolds number (雷诺数) - 3500

§6. Micro-emulsion

Generally : surfactants & cosurfactants and etc

In 1986,the cosurfactant-free microemulsion

have been prepared

• The differentia with macro-emulsion

2. Mechanism of formation

(1) Negative interfacial tension theory – Schulman

and co-workers: γ≤ 0 ∴ spontaneous process

it may be a transient phenomenon(暂时), and at

equilibrium must be zero or slightly positive.

γ is macroscopy property and made no sense in

micro-emulsion

(2) 双重膜(duplex film?)

surfactants + co-surfactants

+ auxiliary agent(助剂)

If γmw > γmo, then the films is bended to water phase, w/o

If γmw < γmo, then the films is bended to oil phase, o/w

(3) Swollen micelles

Mixed film

γmo

γmw

oil

water